run: Teach run to resolve revsets and about jobs.

This also adds `jobs`, the argument reading the thread count to use and `shell_command`.
While we're at it, make `execute` a no-op and teach `run` to resolve the passed revsets. 
I also fixed my misunderstanding of `Clap` which makes 
`jj run 'echo hello world' -r 'mine() & ~origin@remote' --jobs 4` parse correctly. 

Also contains a small fix in the `pre-commit` example for it.

cli/src/commands/run.rs

@@ -12,7 +12,13 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use crate::cli_util::{user_error, CommandError, CommandHelper, RevisionArg};
+//! This file contains the internal implementation of `run`.
+
+use std::num::NonZeroUsize;
+
+use crate::cli_util::{
+    resolve_multiple_nonempty_revsets, user_error, CommandError, CommandHelper, RevisionArg,
+};
 use crate::ui::Ui;
 
 /// Run a command across a set of revisions.
@@ -24,24 +30,41 @@ use crate::ui::Ui;
 /// # Example
 ///
 /// # Run pre-commit on your local work
-/// $ jj run 'pre-commit.py .github/pre-commit.yaml' -r (main..@) -j 4
+/// $ jj run 'pre-commit run .github/pre-commit.yaml' -r (trunk()..@) -j 4
 ///
 /// This allows pre-commit integration and other funny stuff.
 #[derive(clap::Args, Clone, Debug)]
 #[command(verbatim_doc_comment)]
-pub(crate) struct RunArgs {
+pub struct RunArgs {
     /// The command to run across all selected revisions.
-    #[arg(long, short, alias = "x")]
-    command: String,
+    shell_command: String,
     /// The revisions to change.
+    /// Multiple revsets are accepted and the work will be done on a
+    /// intersection of them.
     #[arg(long, short, default_value = "@")]
     revisions: Vec<RevisionArg>,
+    /// A no-op option to match the interface of `git rebase -x`.
+    #[arg(short = 'x', hide = true)]
+    unused_command: bool,
+    /// How many processes should run in parallel, uses by default all cores.
+    #[arg(long, short)]
+    jobs: Option<usize>,
 }
 
-pub(crate) fn cmd_run(
-    _ui: &mut Ui,
-    _command: &CommandHelper,
-    _args: &RunArgs,
-) -> Result<(), CommandError> {
+pub fn cmd_run(ui: &mut Ui, command: &CommandHelper, args: &RunArgs) -> Result<(), CommandError> {
+    let workspace_command = command.workspace_helper(ui)?;
+    let _resolved_commits =
+        resolve_multiple_nonempty_revsets(&args.revisions, &workspace_command, ui)?;
+    let _jobs = if let Some(_jobs) = args.jobs {
+        _jobs
+    } else {
+        // Use all available cores
+
+        // SAFETY:
+        // We use a internal constant of 4 threads, if it fails
+        let available = std::thread::available_parallelism()
+            .unwrap_or(unsafe { NonZeroUsize::new_unchecked(4) });
+        available.into()
+    };
     Err(user_error("This is a stub, do not use"))
 }